This invention relates to a sensor circuit for an AC drive system and is more particularly concerned with a solid state type sensor circuit that will generate an output signal which is proportional to the average value of the applied voltages of one or more electrical phases in the AC drive system.
Previously, adjustable voltage AC drive systems had utilized voltage feedback loops to correct for variations in the incoming line voltages to the controller of the circuit and to compensate for component tolerances within the drive circuits. The voltage feedback sensors incorporated into the older drive systems have traditionally been of the transformer-isolated type and, in these units, only one phase of a three phase voltage being applied to the motor was monitored. Furthermore, in this prior type of sensor circuit, the applied motor stator teminal voltage was stepped down by a transformer, rectified, and then filtered to produce a DC output voltage or current which was proportional to the terminal voltage applied to the motor.
The transformer-isolated sensor circuit had several drawbacks. First, the circuit sensed only one phase of the motor terminal voltage. Secondly, the voltage output of the transformer, once rectified, then needed a substantial amount of filtering to remove the low frequency ripple in order to produce a useable DC signal. Naturally, this type of filtering resulted in a large filtered time constant which further resulted in the response of the sensor circuit to be inherently slow and sluggish which resulted in unstable drive peformance. Next, the cost of the transformer, when compared to other circuit components, was high. In addition, the transformers were bulky, and did not always accurately reproduce the motor stator terminal voltage due to the high harmonic content of the voltage waveform.
Practically, all of these older type of sensor circuits required a number of components that are undesirably large for some applications, and it was in an effort to provide a sensor circuit with better operating capability and with simplier and less costly circuit components that the present invention came about.